1. Field of the Invention
This invention relates to a low consistency one-part silicone composition that is curable by the combination of exposure to moisture, then to heat, to produce a silicone elastomer.
2. Background Information
Silicone compositions have been produced having a viscosity that is sufficiently low so that the composition can be poured or easily extruded into place and then cured. The basic polymer of such compositions has a short chain length in order to be of sufficiently low viscosity to allow the finished composition to be pourable or easily extruded. Such short chain length polymers do not yield cured products with sufficiently high tensile strength and elongation for many applications.
One method of solving this problem has been to use a polymer with mono-functional end groups in conjunction with another difunctional material with which it can react to produce a longer chain polymer. Before reaction, the mixture has a low viscosity; after reaction, the resulting polymer has a higher viscosity. This concept has been further refined by also adding a polyfunctional material to cause the polymer mixture to become crosslinked, as well as chain extended.
Silicone compositions which are in two-part packages that cure when mixed together have been produced. These are not storable as a one-part system. Because the mixing must be done at the time of use, the manufacturer does not have control of the process. Once mixed, the material will cure if not used immediately, and product may be wasted. For these reasons, one-part, storable systems were developed which cure when exposed to moisture. In U.S. Pat. No. 3,766,127, issued Oct. 16, 1973, Clark et al. disclose a composition consisting essentially of a hydroxyl endblocked polydiorganosiloxane, a difunctional amido silane, and a trifunctional amido silane. When the composition is exposed to moisture at room temperature, the ingredients react to produce a low modulus silicone elastomer. Similar compositions are disclosed in U.S. Pat. No. 3,766,128, issued Oct. 16, 1973, by Brady and Clark using a trifunctional amino silane as the crosslinking ingredient and in U.S. Pat. No. 3,817,909, issued June 13, 1974, by Toporcer and Crossan using a polyfunctional aminoxy silane compound The reaction rates between the various reactive groups must be carefully selected in order to produce a cured product that is both crosslinked and chain extended.
In U.S. Pat. No. 4,020,044, issued Apr. 26, 1977, Crossan et al. show that hydroxyl endblocked polydiorganosiloxane can be reacted with a difunctional silane such as methylvinyldi(N-methylacetamido)silane to yield higher molecular weight products. The higher molecular weight product can then be compounded with filler and a vinyl specific organic peroxide to provide stocks which can be heat cured. They do not teach that a one-part, storage stable composition can be produced that can then be cured by exposure to moisture followed by exposure to heat.